Coins have been made from nickel plated on steel, but there is a tendency for rust spots to develop at pinhole locations where the plating does not totally cover the steel. Pinholes may occur in the plated layer as a result of surface phenomenon in the layer of nickel plating or may be the result of micropores at the surface of the steel.
During coining, the dies stretch the metal, particularly at the edge of the coin. Pinholes may extend to expose the steel as a result of this stretching. Cracks in the plating may also develop at the edges. Either of these occurrences will result in rust.
If there are pores in the plating and these are bridged during the electroplating process, the entrapped air may blow during the coining procedure causing blisters. This is a severe problem in coinage. Some manufacturers of coins have been known to pound the metal with small steel balls to tray to minimize the problem of blisters and pinholes.
Another problem that develops during coining is known as `starbursts`. During nickel plating the layer of nickel would build up peaks. During coining these spots will be cut off or flattened. This abrasive action would score the surface of the dies.